A conventional gas turbine includes a rotor with various rotor blades mounted to rotor disks in the fan, compressor, and turbine sections thereof. Each blade includes an airfoil over which the pressurized air flows, and a platform at the root of the airfoil that defines the radially inner boundary for the airflow. The blades are typically removable, and therefore include a suitable dovetail configured to engage a complementary dovetail slot in the perimeter of the rotor disk. The dovetails may either be axial-entry dovetails or circumferential-entry dovetails that engage corresponding axial or circumferential slots formed in the disk perimeter. A typical dovetail includes a neck of minimum cross sectional area extending radially inwardly from the bottom of the blade platform. The neck diverges outwardly into a pair of opposite dovetail lobes.
Components of a conventional gas turbine are illustrated, for example, in FIG. 1 wherein a rotor 12 includes a plurality of rotor disks 20 disposed coaxially with the centerline axis 18 of the turbine. A plurality of circumferentially spaced rotor blades 22 are removably fixed to the disk and extend radially outward therefrom. Each blade 22 has a longitudinal centerline axis 24 and includes an airfoil section 26 having a leading edge 26a and a trailing edge 26b (in the direction of airflow over the blade 22). Each blade 22 has a platform 28 that provides a portion of the radially inner boundary for the airflow over the airfoils 26, and an integral dovetail 30 that extends radially inward from the platform 28 and is configured for axial entry into circumferentially spaced apart and axially extending dovetail slots defined between corresponding disk posts in the rotor disk 20. The axial slots and disk posts extend essentially the full axial thickness of the disk between its axially forward and aft faces.
For circumferential dovetails, a single dovetail slot is formed between forward and aft continuous circumferential posts or “hoops” and extends circumferentially around the entire perimeter of the disk. An example of this type of configuration is shown in U.S. Pat. No. 6,033,185. The circumferential slot may be locally enlarged at one location for allowing the individual circumferential dovetails to be initially inserted therein and then repositioned circumferentially along the dovetail slot until the entire slot is filled with a full row of the blades. In an alternate conventional configuration, the circumferential slot is provided with circumferentially spaced load-lock slots, as depicted in FIG. 2 of this application. Referring to FIG. 2, the rotor disk 20 has a continuous circumferential slot 18 defined between continuous hoops 20, 22. Loading slots 14 are provide for initial insertion and rotation of individual rotor blade dovetails. Lock slots 16 are provided for insertion of locks to retain the blades in the slot 18.
In the circumferential dovetail slot, the forward and aft hoops include complementary lobes that cooperate with the dovetail lobes to radially retain the individual blades against centrifugal force during turbine operation. Each dovetail lobe includes a radially outwardly facing outer pressure surface or face that engages a corresponding radially inwardly facing pressure surface or face of the respective disk post. The centrifugal load generated by the blade during rotation is carried radially outward from the dovetail lobes and transferred to the respective disk posts at the engaging outer (dovetail lobe) and inner (disk post) pressure faces.
For the blade dovetails, maximum centrifugal stress is experienced at the necks, which stress must be limited by design to ensure blade life. A typical compressor blade is designed for an infinite life, which requires suitably large dovetails and necks for maintaining centrifugal stress suitably below the strength limits of the blade material. For the rotor disks, maximum stress imparted by the centrifugal load of the blades and axial loads is experienced primarily at the dovetail hoops. As generally recognized in the art, the hoop stress for the load-lock slot configuration is more limiting than for a continuous slot configuration since the locking and loading slots form discontinuities that are prone to mechanical and thermal stresses, and fatigue.
Examples of various proposals to reduce stress in dovetail configurations may be found, for example, in U.S. Pat. No. 6,033,185 cited above; U.S. Pat. Nos. 5,310,318; 5,100,292; 5,271,718; 5,584,658; 4,451,203; and U.S. Pat. App. Pub. 2007/0014667.
The art is continuously seeking improved dovetail designs that reduce stress and extend the useful life of rotor components, particularly as the size and demands placed on gas turbines, and resulting stresses, grow.